1. Field of the Invention
The invention concerns a method for the detection of short circuits in electrical installations, in particular on railway contact wires, whereby a measuring signal dependent on the derivative with respect to time of the monitored current is generated and compared with at least one limit value, and whereby the time for which the measuring signal exceeds a limit value is determined and compared with a preset time interval. When the time for which the measuring signal exceeds the limit value also exceeds this time interval, this fact is used to initiate indication of a short circuit. The invention further concerns apparatus for executing such a method.
2. Description of the Prior Art
Methods and apparatus of the kind stated above have already been proposed for application in the monitoring of short circuits on direct-current contact wires of electrical railways, whereby if the measuring signal exceeds a limit value for a length of time greater than a preset time interval, indication of a short circuit is initiated immediately together with disconnection of the affected line section. This technique has proved increasingly unreliable, especially in the case of sections with a high traffic frequency, which can result in current being drawn by a number of traction vehicles on one section of line between two power supply substations. In the event of the starting currents of different traction vehicles, or of the motor sets of multiple compositions, becoming superimposed on each other, this can give rise to current-flow conditions in which the amplitude of the current and its differential decay time, or the time for which it exceeds a limit value, are of an order of magnitude similar to that of short circuits.
The proposed measures therefore no longer allow a sufficiently clear distinction between short circuits and operating conditions which are still acceptable, and lead to unnecessary disconnections and hence interruptions of service.